The present invention relates generally to removing mercury and mercury-containing compounds from liquid wastes and specifically to removing mercury and mercury-containing compounds from dental effluents.
Each year tens of thousands of pounds of mercury-containing wastes are discharged by dental offices into municipal waste systems. Amalgam fillings typically contain about 50% mercury by weight. Mercury is a known environmental contaminant, classified by the USEPA as a persistent, bioaccumulative, and toxic material. Waste water treatment plants must meet strict limits on the amount of mercury they can release. The discharged form of mercury is typically highly toxic (i.e., unstable) and in violation of applicable environmental regulations. Although particulate removal systems used in some dental offices remove mercury-containing particles, they do not remove dissolved mercury and mercury-containing compounds. Examples of such devices are described in U.S. Pat. Nos. 5,885,076; 5,797,742; 5,795,159; 5,577,910; 5,227,053; 4,753,632; 4,591,437; 4,385,891; and 5,114,578, all of which are incorporated herein by this reference.
The present invention provides a method and apparatus for removing solid amalgam particles and/or soluble forms of mercury and other metals (such as silver) and other contaminants from dental effluents.
In one embodiment, a contaminant removal system is provided for treating a three-phase effluent. The system includes:
(a) a particle collection vessel for separating a gas phase, a liquid phase, and a solid (particulate) phase in the three-phase of effluent;
(b) a first discharge line from the vessel for removing the liquid phase from the vessel;
(c) a second discharge line from the vessel for removing the gas phase from the vessel; and optionally
(d) a liquid treatment device for removing and/or stabilizing contaminants in the liquid phase to form a treated liquid phase. The contaminant can be any undesirable organic or inorganic material in the effluent. Examples include metals (e.g., mercury, lead, arsenic, etc.), metal compounds, bacteria, pathogens, inorganic and organic solvents, and mixtures thereof.
The particle collection vessel can be of any suitable design. For example, the vessel can be a settling tank, a sedimentation device, a centrifuge, or any other device that utilizes gravity or centrifugal forces for effectuating solids/liquid separation. In one configuration, the particle collection vessel includes one or more baffles to facilitate solid/liquid/gas separation.
To remove the liquid phase from the particle collection vessel, the system can include a timer connected to a pump on the first discharge line to cause periodic removal of the liquid phase from the vessel during periods when no waste is incoming (e.g., overnight). In this manner, the liquid is provided with a relatively quiescent period for effective settlement of entrained particles such as amalgam particles.
The liquid treatment device can be any suitable device for removing and/or stabilizing contaminants in the liquid phase to form the treated liquid phase. In one configuration, the liquid treatment device includes one or more devices for adding chemical additives, such as one or more of a pH adjustor, an oxidant, a reductant, and a precipitant with the liquid phase. In one configuration, the liquid treatment device includes one or more sorbent columns that contact the liquid phase with one or more sorbents.
The effluent can be any contaminated effluent regardless of the source. In a preferred configuration, the effluent is produced by dental work on a patient. The effluent is collected by a liquid collection device, such as a sink, suction tube or, evacuation line, and conveyed to the particle collection vessel via a waste discharge line. A single particle collection vessel can service a plurality of such liquid collection devices corresponding to a plurality of dental chairs.
In another embodiment, a process is provided for removing dissolved contaminants from the three-phase effluent. The process includes the steps of:
(a) introducing the three-phase effluent into a particle collection vessel;
(b) reducing the velocity of and/or redirecting the direction of movement of the effluent, thereby causing a solid phase and a liquid phase to separate from a gas phase;
(c) removing the gas phase from the collection vessel;
(d) removing the liquid phase from the collection vessel; and optionally
(e) contacting the liquid phase with at least one of an additive and a sorbent to form a treated liquid phase.
As noted, the liquid phase can be removed discontinuously from the collection vessel to provide more effective separation of the entrained particles from the liquid phase.
In one configuration, the contacting step further includes the steps of;
(f) first contacting the liquid phase with one or more of a reductant, an oxidant, a participant, and a pH adjustor I (typically prior to removal of the liquid phase); and
(g) second contacting the liquid phase with a sorbent to remove mercury therefrom. In another configuration, step (g) is optional.
As will be appreciated, the additive, preferably a reactant (a compound that will react with the contaminant, such as a reductant, oxidant, and/or precipitant), may be used in the absence of (or without) a sorbent. For example, the reactant could be a precipitant that forms a precipitate with the contaminant. The precipitate could be removed from the effluent by filtration techniques, gravity separation techniques, etc. A flocculent, such as aluminum or commercially available polymers, could be further added to the effluent to act as a filter and/or settling aid.
The system and method can provide numerous benefits. For example, the system can remove not only solid amalgam particles but also remove and/or stabilize dissolved elemental mercury and speciated mercury. The system and method can be effective at capturing a high percentage of the amalgam particles. The vessel typically captures or collects at least most and more typically at least about 95% of all amalgam particles that are about 10 microns or greater in size. The chemical treatment device can further remove at least most of the amalgam particles that are less than about 10 microns in size. Amalgam particles typically represent at least 95% of the total mercury sent to the system. Any of the system components can be used as a recycling device. For example, the collection vessel or sorbent column can be operated for a predetermined period (typically 6-12 months) after which the vessel and/or column is replaced. The used vessel and/or column is shipped to a recycling facility to recover the captured amalgam particles and/or elemental and speciated mercury. The system can operate effectively without a sorbent. Proper selection of the additives can remove the need for a sorbent.